This invention relates to a method and system for determining the effective ferric ion-complexing capacity of a hydrofluoric acid-containing cleaning liquid. The invention is particularly well adapted for use with mixed acid (hydrofluoric and nitric acids) pickling liquors, such as those used in steel processing.
Various grades of steel, including stainless steel, are cleaned during processing with a pickling liquor to remove oxides and scale from the surface of the metal. Pickling liquors typically comprise a concentrated, mixed acid solution of hydrofluoric acid (HF) and nitric acid (HNO.sub.3).
During a pickling process nitric acid, a strong oxidizer, is used to dissolve oxides and scale from the metal surface. These are primarily oxides of iron in stainless steel, but are also oxides of chromium, nickel and minor amounts of other metals. Fluoride, in the form of hydrogen fluoride, is added to the bath to complex these metal ions as they are dissolved. In the absence of fluorides, a passive layer would form on the steel due to hydrolysis, and pickling would quickly cease. Hydrogen fluoride is able to hold these metals in solution through the formation of metal complexes represented by the general formulas H.sub.3 MF.sub.6 and H.sub.2 MF.sub.5, where M is a metal such as Fe, Ni or Cr. In addition, siliceous scale, containing non-metallic silica, is also cleaned through the formation of soluble silicon fluoride complexes.
Optimum concentration levels of acids and of fluoride, as HF, in a pickling liquor are to be maintained to ensure proper and efficient pickling. Thus, frequent monitoring of the pickling bath is desirable. However, there is no standard analytical method for monitoring the condition of a pickling bath. Indeed, controversy exists as to whether the proper species to analyze is free fluoride ion, total fluoride, hydrogen fluoride concentration, or combinations thereof.
Accurate analysis of the pickling bath is difficult because free fluoride ion (F.sup.-) is not present in a measurable quantity due to the highly acidic nature of the bath. As pickling progresses, HF, the active complexer, is consumed with the formation of complexes. Analytical methods which seek to measure the concentration of fluoride ion require an upward adjustment of the pH of the sample. However, such techniques foster inaccurate results as the pH adjustment disturbs the chemical equilibrium relationships that exist in the bath. For example, if the pH is adjusted to 4, HF dissociates to F.sup.-, and complexed metal, as H.sub.3 MF.sub.6, is converted to hydrolytic compounds such as M(OH).sub.x, while HF is liberated. Analysis of HF under these conditions thus gives inaccurate results because the pH change causes the sample being analyzed to contain more HF than the actual pickling bath. Data obtained through such techniques are thus not relevant to the actual pickling liquor. Moreover, such techniques use "grab samples", which are pickling liquor samples removed from the pickling bath and analyzed off-line, often at a remote location. These methods do not provide "real time" information as desired to assess the ever changing conditions of an industrial pickling line.
Accordingly, there is a need for a real-time method of analyzing the condition of a mixed acid liquid, such as a pickling bath, to provide reliable and accurate information on the ability of the bath to continue a pickling process at an acceptable rate.
It is thus an object of the invention to provide an improved method for determining the iron-complexing power of a pickling bath. Another object of the invention is to provide an analytical method for performing an analysis of the iron-complexing power of a pickling bath which does not require a significant change in the pH of the sample. It is also an object of the invention to perform the analysis in an on-line manner, and to obtain data in essentially real-time. A further object of the invention is to provide a system for performing on-line, automated measurements of the iron-complexing power of a pickling liquor. Other objects of the invention will be apparent from the disclosure which follows.